Ring Electrode for Fluid Ejection

ABSTRACT

Methods, systems, and apparatus for drive a pumping chamber of a fluid ejection system are disclosed. In one implementation, the actuator for drive the pumping chamber includes a continuous piezoelectric layer between a pair of drive electrodes and a continuous reference electrode. The pair of drive electrodes includes an inner electrode and an outer electrode surrounding the inner electrode. The actuator is further coupled to a controller which, during a fluid ejection cycle, applies a negative voltage pulse differential to the outer electrode to expand the pumping chamber for a first time period, then applies another negative voltage pulse differential to the inner electrode during a second time period after the first time period to contract the pumping chamber to eject a fluid drop.

PRIORITY INFORMATION

This is a divisional application and claims the priority of U.S.application Ser. No. 13/275,201, filed Oct. 17, 2011, which is adivisional application of U.S. application Ser. No. 12/389,317, filedFeb. 19, 2009, now issued as U.S. Pat. No. 8,061,820. The disclosures ofthe prior applications are considered part of this application and areincorporated by reference herein in their entireties.

TECHNICAL FIELD

This specification relates to piezoelectrically-actuated fluid ejection.

BACKGROUND

A fluid ejection system typically includes a fluid path from a fluidsupply to a nozzle assembly that includes nozzles from which fluid dropsare ejected. Fluid drop ejection can be controlled by pressurizing fluidin the fluid path with an actuator, such as a piezoelectric actuator.The fluid to be ejected can be, for example, an ink, electroluminescentmaterials, biological compounds, or materials for formation ofelectrical circuits.

A printhead module in an ink jet printer is an example of a fluidejection system. A printhead module typically has a line or an array ofnozzles with a corresponding array of ink paths and associatedactuators, and drop ejection from each nozzle can be independentlycontrolled by one or more controllers. The printhead module can includea semiconductor printhead body in which the ink paths are formed andpiezoelectric actuators attached to the printhead body. A nozzle can bedefined by a separate layer that is attached to the printhead body. Theprinthead body can be made of a silicon substrate etched to define apumping chamber along an ink path. One side of the pumping chamber is amembrane that is sufficiently thin to flex and expand or contract thepumping chamber when driven by the piezoelectric actuator. Thepiezoelectric actuator is supported on the membrane over the pumpingchamber. The piezoelectric actuator includes a layer of piezoelectricmaterial that changes geometry (or actuates) in response to a voltageapplied across the piezoelectric layer by a pair of opposing electrodes.The actuation of the piezoelectric layer causes the membrane to flex,and flexing of the membrane thereby pressurizes ink in the pumpingchamber along the ink path and eventually ejects an ink droplet out ofthe nozzle.

SUMMARY

This specification describes technologies related to fluid ejection. Ingeneral, a piezoelectric actuator that includes a ring electrode andpiezoelectric material with a uniform poling direction can be actuatedwith a unipolar voltage waveform.

In one aspect, a fluid ejection system includes a substrate having achamber formed therein, a membrane that forms a wall of the chamber andis operable to expand or contract the chamber by flexing, and anactuator supported on the membrane. The actuator includes apiezoelectric layer disposed between a drive electrode layer and areference electrode layer. The piezoelectric layer includes a continuousplanar piezoelectric material spanning the chamber and having a uniformpoling direction substantially perpendicular to the continuous planarpiezoelectric material. The drive electrode layer includes a pluralityof drive electrodes, the plurality of drive electrodes including aninner electrode and an outer electrode surrounding the inner electrode.The reference electrode layer includes a reference electrode having afirst portion spanning the inner electrode and a second portion spanningthe outer electrode. Creation of a voltage differential between at leastone of the plurality of drive electrodes and the reference electrodegenerates an electric field in the continuous planar piezoelectricmaterial, and the electric field results in actuation of the continuousplanar piezoelectric material to flex the membrane.

Implementations may include one or more of the following features. Acontroller may be electrically coupled to the plurality of driveelectrodes and the reference electrode, and during an operation cycle toeject a fluid droplet, the controller may be operable to create a firstvoltage differential pulse between a first electrode in the plurality ofdrive electrodes and the reference electrode during a first time periodto expand the chamber, and to create a second voltage differential pulsebetween a second electrode in the plurality of drive electrodes and thereference electrode during a second time period to contract the chamber,the second electrode being different from the first electrode and thesecond time period being after the first time period. The first voltagedifferential pulse and the second voltage differential pulse may eachgenerate an electric field that points in substantially the samedirection as the poling direction.

The first electrode in the plurality of drive electrode may be the outerelectrode, and the second electrode in the plurality of drive electrodemay be the inner electrode.

The membrane may be a separate layer bonded to the substrate.

The inner electrode may be disposed over a central portion of themembrane and the outer electrode may be disposed over a peripheralportion of the membrane surrounding the inner electrode.

The inner electrode and the outer electrode may be the only driveelectrodes disposed over the membrane.

The membrane may have a lateral dimension of D, and the inner electrodemay have a lateral dimension of approximately ⅔ of D. The outerelectrode may be in the shape of a ring, where the ring has an innerlateral dimension and an outer lateral dimension, and the inner lateraldimension of the ring may be greater than the lateral dimension of theinner electrode. The outer lateral dimension of the ring may be greaterthan the lateral dimension of the membrane. The lateral dimension of theinner electrode and the inner lateral dimension of the outer electrodemay be such that a maximum volume displacement is achieved betweenexpansion and contraction of the chamber. The lateral dimension of theinner electrode and the inner lateral dimension of the outer electrodemay also be such that equal volume displacement is achieved betweenexpansion and contraction of the chamber.

The continuous planar piezoelectric material may be a lead zirconatetitanate (PZT) film.

In one aspect, a fluid ejection system includes a substrate having achamber formed therein, a membrane that forms a wall of the chamber andis operable to expand or contract the chamber by flexing, and anactuator supported on the membrane. The actuator includes apiezoelectric layer disposed between a drive electrode layer and areference electrode layer. The piezoelectric layer includes a continuousplanar piezoelectric material spanning the chamber and having a uniformpoling direction that is substantially perpendicular to the continuousplanar piezoelectric material. The drive electrode layer includes adrive electrode in contact with the continuous planar piezoelectricmaterial. The drive electrode is in the shape of a ring, disposed over aperipheral portion of the membrane, and being the sole drive electrodeover the membrane in the drive electrode layer. The reference electrodelayer includes a reference electrode spanning at least an area coveredby the drive electrode. Creation of a voltage differential between thedrive electrode and the reference electrode generates an electric fieldin the continuous planar piezoelectric material, and the electric fieldresults in actuation of the continuous planar piezoelectric material toflex the membrane.

Implementations may include one or more of the following features. Thefluid ejection system may further include a controller electricallycoupled to the drive electrode and the reference electrode. During anoperation cycle to eject a fluid droplet, the controller may be operableto create a voltage differential between a drive electrode and thereference electrode for a time period to expand the chamber, and toremove the voltage differential after the time period to contract thechamber. The uniform poling direction of the continuous planarpiezoelectric material may point from the reference electrode layer tothe drive electrode layer, and the voltage differential may be anegative voltage differential. And at the same time, the uniform polingdirection of the continuous planar piezoelectric material may pointsaway from the chamber.

The membrane may be a lateral dimension of D. The drive electrode mayhave an inner lateral dimension and an outer lateral dimension, and theouter lateral dimension of the drive electrode may be greater than thelateral dimension of the membrane. The inner lateral dimension of thedrive electrode may be such that a maximum volume displacement isachieved between expansion and contraction of the chamber. Thecontinuous planar piezoelectric material may be a lead zirconatetitanate (PZT) film.

In one aspect, a fluid ejection system includes a substrate having achamber formed therein, a membrane that forms a wall of the chamber andis operable to expand or contract the chamber by flexing, and anactuator supported on the membrane. The actuator includes apiezoelectric layer disposed between a drive electrode layer and areference electrode layer. The piezoelectric layer includes a planarpiezoelectric material spanning the chamber and having a uniform polingdirection substantially perpendicular to the planar piezoelectricmaterial. The drive electrode layer includes a plurality of driveelectrodes. The plurality of drive electrodes includes an innerelectrode and an outer electrode surrounding the inner electrode. Thereference electrode layer includes a reference electrode, the referenceelectrode having a first portion spanning the inner electrode and asecond portion spanning the outer electrode. The fluid ejection systemfurther includes a controller electrically coupled to the pair of driveelectrodes and the reference electrode. During an operation cycle toeject a fluid droplet, the controller is operable to create a firstvoltage differential pulse between a first electrode in the plurality ofdrive electrodes and the reference electrode during a first time periodto expand the chamber. The controller is further operable to create asecond voltage differential pulse between a second electrode in theplurality of drive electrodes and the reference electrode during asecond time period to contract the chamber, the second electrode beingdifferent from the first electrode and the second time period beingafter the first time period.

Implementations may include one or more of the following features. Thefirst voltage differential pulse and the second voltage differentialpulse may each generate an electric field that points substantially inthe same direction as the poling direction. The first electrode in theplurality of drive electrode may be the outer electrode, and the secondelectrode in the plurality of drive electrode may be the innerelectrode.

The uniform poling direction of the planar piezoelectric material maypoint from the reference electrode layer to the drive electrode layer,the first voltage differential pulse may be a negative voltagedifferential pulse, and the second voltage differential pulse may beanother negative voltage differential pulse. The uniform polingdirection of the planar piezoelectric material may point away from thechamber. The uniform poling direction of the planar piezoelectricmaterial may point toward the chamber.

The uniform poling direction of the planar piezoelectric material maypoint from the drive electrode layer to the reference electrode layer,the first voltage differential pulse may be a positive voltagedifferential pulse, and the second voltage differential pulse may beanother positive voltage differential pulse. The uniform polingdirection of the planar piezoelectric material may point away from thechamber. The uniform poling direction of the planar piezoelectricmaterial may point toward the chamber.

The first voltage differential pulse and the second voltage differentialpulse may each generate an electric field that points in a directionsubstantially opposite to the poling direction. The first electrode inthe plurality of drive electrode is the inner electrode, and the secondelectrode in the plurality of drive electrode is the outer electrode.

The uniform poling direction of the planar piezoelectric material maypoint from the reference electrode layer to the drive electrode layer,the first voltage differential pulse may be a positive voltagedifferential pulse, and the second voltage differential pulse may beanother positive voltage differential pulse. The uniform polingdirection of the planar piezoelectric material may point away from thechamber. The uniform poling direction of the planar piezoelectricmaterial may point toward the chamber.

The uniform poling direction of the planar piezoelectric material maypoint from the drive electrode layer to the reference electrode layer,the first voltage differential pulse may be a negative voltagedifferential pulse, and the second voltage differential pulse may beanother negative voltage differential pulse. The uniform polingdirection of the planar piezoelectric material may point away from thechamber. The uniform poling direction of the planar piezoelectricmaterial may point toward the chamber.

In one aspect, a fluid ejection system includes a substrate having achamber formed therein, a membrane that forms a wall of the chamber andis operable to expand or contract the chamber by flexing, and anactuator supported on the membrane. The actuator includes apiezoelectric layer disposed between a drive electrode layer and areference electrode layer. The piezoelectric layer includes a planarpiezoelectric material spanning the chamber and having a uniform polingdirection that is substantially perpendicular to the planarpiezoelectric material. The drive electrode layer includes a driveelectrode in contact with the planar piezoelectric material. The driveelectrode is in the shape of a ring, disposed over a peripheral portionof the membrane, and being the sole drive electrode over the membrane inthe drive electrode layer. The reference electrode layer includes areference electrode spanning at least an area covered by the driveelectrode. The fluid ejection system further includes a controllerelectrically coupled to the drive electrode and the reference electrode.During an operation cycle to eject a fluid droplet, the controller isoperable to create a voltage differential between a drive electrode andthe reference electrode for a time period to expand the chamber, and toremove the voltage differential after the time period to contract thechamber.

Implementations may include one or more of the following features. Theuniform poling direction of the planar piezoelectric material may pointfrom the reference electrode layer to the drive electrode layer. Thevoltage differential may be a negative voltage differential. The uniformpoling direction of the planar piezoelectric material may points awayfrom the chamber. The uniform poling direction of the planarpiezoelectric material may point toward the chamber.

The uniform poling direction of the planar piezoelectric material maypoint from the drive electrode layer to the reference electrode layer.The voltage differential may be a positive voltage potential. Theuniform poling direction of the planar piezoelectric material may pointaway from the chamber. The uniform poling direction of the planarpiezoelectric material may point toward the chamber.

In one aspect, a method for actuating a pumping chamber includesgenerating a negative voltage differential for a time period between adrive electrode and a reference electrode of a piezoelectric actuator toexpand the pumping chamber, and removing the negative voltagedifferential after the time period to contract the pumping chamber. Thepumping chamber is a cavity formed in a substrate. The cavity is coveredby a membrane that is operable to expand or contract the pumping chamberby flexing. The piezoelectric actuator is supported on the membrane andincludes a planar piezoelectric material, the drive electrode and thereference electrode. The drive electrode and the reference electrode aredisposed on opposite sides of the planar piezoelectric material. Thedrive electrode is in a shape of a ring, disposed over a peripheralportion of the membrane, and being the sole drive electrode over themembrane. The planar piezoelectric material spans the chamber and has auniform poling direction that is substantially perpendicular to theplanar piezoelectric material and points away from the pumping chamber.The reference electrode is disposed between the membrane and the driveelectrode. The reference electrode spans at least an area covered by thedrive electrode.

In one aspect, a fluid ejection system includes a substrate having achamber and a membrane. The membrane covers one side of the chamber andis operable to expand or contract the chamber by flexing. The fluidejection system further includes an actuator on the membrane. Theactuator includes a piezoelectric layer disposed between a driveelectrode layer and a reference electrode layer. The piezoelectric layerhas a uniform poling direction substantially perpendicular to a surfaceof the membrane in contact with the actuator. The drive electrode layerincludes a plurality of drive electrodes. The plurality of driveelectrodes includes an inner electrode and an outer electrodesurrounding the inner electrode. The fluid ejection system furtherincludes a controller electrically coupled to the actuator to provide aunipolar waveform to the inner electrode and outer electrode.

Implementations may include one or more of the following features.During an operation cycle to eject a fluid droplet, the controller maybe operable to create a first voltage differential pulse between a firstelectrode in the plurality of drive electrodes and the referenceelectrode during a first time period to expand the chamber, and tocreate a second voltage differential pulse between a second electrode inthe plurality of drive electrodes and the reference electrode during asecond time period to contract the chamber. The second electrode may bedifferent from the first electrode and the second time period may beafter the first time period. The first voltage differential pulse andthe second voltage differential pulse may each generate an electricfield that is substantially in the same direction as the polingdirection. The first electrode in the plurality of drive electrode maybe the outer electrode, and the second electrode in the plurality ofdrive electrode may be the inner electrode.

In one aspect, a fluid ejection system includes a substrate having achamber and a membrane. The membrane covers one side of the chamber andis operable to expand or contract the chamber by flexing. The fluidejection system further includes an actuator on the membrane. Theactuator includes a piezoelectric layer disposed between a driveelectrode layer and a reference electrode layer. The piezoelectric layerhas a uniform poling direction substantially perpendicular to a surfaceof the membrane in contact with the actuator. The drive electrode layerincludes a drive electrode in contact with the piezoelectric material.The drive electrode is ring-shaped and disposed over a peripheralportion of the membrane over the chamber, and being the sole driveelectrode over the membrane in the drive electrode layer. The fluidejection system further includes a controller electrically coupled tothe actuator to provide a unipolar waveform to the drive electrode.

The unipolar waveform applied to the actuator may create an electricalfield in the piezoelectric layer that is substantially in the samedirection as the poling field.

In one aspect, a fluid ejection system includes a substrate having achamber and a membrane. The membrane covers one side of the chamber andis operable to expand or contract the chamber by flexing. The fluidejection system further includes an actuator on the membrane. Theactuator includes a piezoelectric layer disposed between a driveelectrode layer and a reference electrode layer. The piezoelectric layerhas a uniform poling direction substantially perpendicular to a surfaceof the membrane in contact with the actuator. The drive electrode layerincludes a plurality of drive electrodes. The plurality of driveelectrodes includes an inner electrode and an outer electrodesurrounding the inner electrode. The fluid ejection system furtherincludes a controller electrically coupled to the actuator to provide aunipolar waveform to the inner electrode and outer electrode.

Implementations may include one or more of the following features.During an operation cycle to eject a fluid droplet, the controller maybe operable to create a first voltage differential pulse between a firstelectrode in the plurality of drive electrodes and the referenceelectrode during a first time period to expand the chamber, and tocreate a second voltage differential pulse between a second electrode inthe plurality of drive electrodes and the reference electrode during asecond time period to contract the chamber, the second electrode beingdifferent from the first electrode and the second time period beingafter the first time period. The first voltage differential pulse andthe second voltage differential pulse may each generate an electricfield that is substantially in the same direction as the polingdirection. The first electrode in the plurality of drive electrode maybe the outer electrode, and the second electrode in the plurality ofdrive electrode may be the inner electrode.

In one aspect, a fluid ejection system includes a substrate having achamber and a membrane. The membrane covers one side of the chamber andis operable to expand or contract the chamber by flexing. The fluidejection system further includes an actuator on the membrane. Theactuator includes a piezoelectric layer disposed between a driveelectrode layer and a reference electrode layer. The piezoelectric layerhas a uniform poling direction substantially perpendicular to a surfaceof the membrane in contact with the actuator. The drive electrode layerincludes a drive electrode in contact with the piezoelectric material.The drive electrode is ring-shaped and disposed over a peripheralportion of the membrane over the chamber, and being the sole driveelectrode over the membrane in the drive electrode layer. The fluidejection system further includes a controller electrically coupled tothe actuator to provide a unipolar waveform to the drive electrode.

The unipolar waveform applied to the actuator may create an electricalfield in the piezoelectric layer that is substantially in the samedirection as the poling field.

The technology disclosed can offer one or more of the followingadvantages. A ring-shaped drive electrode can eliminate the need for apositive drive voltage in a fluid ejection cycle and the need formaintaining a quiescent negative bias while idle. A dual electrodedesign can permit the piezoelectric layer in the actuator to remain in aneutral and relaxed state rather than a pre-compressed state while idle.This can reduce fatigue of the piezoelectric material. For example, onlynegative drive voltages need be applied across the piezoelectric layerduring each fluid ejection cycle. The electric field created inside thepiezoelectric layer can point in the same direction as the polingdirection of the piezoelectric material during the entire fluid ejectioncycle. Fatigue and depolarization of the piezoelectric material in thepiezoelectric actuator can be reduced.

A dual electrode design can provide a greater total volume displacementfor a given drive voltage as compared to a single electrode design.Alternatively, for a given volume displacement required during a fluidejection cycle, the drive voltage can be reduced roughly by half ascompared to a single electrode design.

An increase in volume displacement efficiency due to the dual electrodedesign can allow for a smaller actuator membrane to achieve sufficientvolume displacement for fluid ejection, and nozzle density can beincreased. In addition, the reduced membrane area can also improvemembrane strength for fluid ejection.

Other aspects, features, and advantages will be apparent from thedescription and drawings and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an exemplary fluidejection system.

FIG. 2A is a schematic cross-sectional view of an exemplary fluidejection system with a central drive electrode.

FIG. 2B is a schematic top view of an exemplary fluid ejection systemwith a central drive electrode.

FIG. 2C is an exemplary drive voltage waveform for a droplet ejectioncycle in the fluid ejection system of FIGS. 2A-2B.

FIG. 2D is another exemplary drive voltage waveform for a dropletejection cycle in the fluid ejection system of FIGS. 2A-2B.

FIG. 3A is a schematic cross-sectional view of an exemplary fluidejection system with a ring-shaped drive electrode.

FIG. 3B is a schematic top view of an exemplary fluid ejection systemwith a ring-shaped drive electrode.

FIG. 3C is an exemplary drive voltage waveform for a droplet ejectioncycle in the fluid ejection system of FIGS. 3A-3B.

FIG. 4A illustrates the deflection of a piezoelectric membrane under apositive voltage applied between a central drive electrode and acontinuous reference electrode.

FIG. 4B illustrates the deflection of a piezoelectric membrane under apositive voltage applied between a ring-shaped drive electrode and areference electrode.

FIG. 5A is a schematic diagram of a dual electrode piezoelectricactuator including an inner drive electrode (e.g., a central electrode)and an outer drive electrode (e.g., a ring-shaped electrode) surroundingthe inner drive electrode.

FIG. 5B illustrates exemplary drive voltage waveforms on the centralelectrode and the ring-shaped electrode for a droplet ejection cycle ina fluid ejection system utilizing the dual electrode piezoelectricactuator of FIG. 5A.

FIG. 6 is a plot illustrating the correlations between volumedisplacement per volt and the lateral dimensions of the central and thering-shaped drive electrodes in relation to the lateral dimension of thepumping chamber.

FIG. 7 is a plot illustrating the correlation between volumedisplacement per volt (dV/dU) in the pumping chamber and percentage ofdrive electrode radius relative to pumping chamber radius (%).

FIG. 8A is a piezoelectric actuator with an exemplary ring-shapedelectrode over a continuous PZT layer.

FIG. 8B illustrates the deflection of the PZT layer in FIG. 8A under anapplied voltage between the ring-shaped electrode and a referenceelectrode.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 is a schematic cross-sectional view of an exemplary fluidejection system (e.g., a printhead module 100).

The printhead module 100 includes a plurality of piezoelectric actuatorstructures 120 and a module substrate 110 through which a plurality ofpassages are formed.

The module substrate 110 can be a monolithic semiconductor body such asa silicon substrate. Each passage through the silicon substrate definesa flow path for the fluid (e.g., ink) to be ejected (only one flow pathand one actuator are shown in the cross-sectional view of FIG. 1). Eachflow path can include a fluid inlet 112, a pumping chamber 114, adescender 116, and a nozzle 118. The flow path can further include arecirculation path 119 so that ink can flow through the flow path evenwhen fluid is not being ejected through the nozzle 118. The pumpingchamber 114 is a cavity formed in the module substrate 110. One side ofthe pumping chamber 114 is a membrane 123 that is capable of flexing toexpand or contract the pumping chamber 114 and pump the fluid along theflow path. The area of the membrane 123 can be defined by the edge ofthe pumping chamber 114 supporting the membrane 123. Stated another way,the lateral shape of the membrane 123 is defined by the perimeter of thepumping chamber 114 under the membrane. The membrane 123 can be formedby joining (e.g., bonding, for example, silicon-to-silicon fusionbonding) a membrane layer 122 to the module substrate 110 over an openside of the pumping chamber 114. Alternatively, the membrane 123 can bea thin portion of the monolithic semiconductor substrate that was leftintact during the etching of the pumping chamber 114. The piezoelectricactuator structure 120 is positioned over the pumping chamber 114 andsupported on the exposed side of the membrane 123.

The piezoelectric actuator structure 120 includes a first electrodelayer (e.g., a reference electrode layer 124, e.g., connected toground), a second electrode layer (e.g., a drive electrode layer 128),and a piezoelectric layer 126 disposed between the first and the secondelectrode layers. The piezoelectric actuator structure 120 is supportedon (e.g., bonded to) to the module substrate 110 containing the membrane123. In some implementations, the piezoelectric actuator structure 120is fabricated separately and then secured, (e.g., bonded) to the modulesubstrate 110. In some implementations, the piezoelectric actuatorstructure 120 can be fabricated in place over the pumping chamber bysequentially deposition of layers onto the module substrate 110 (or themembrane layer 122).

The piezoelectric layer 126 changes geometry, or bends, in response to avoltage applied across the piezoelectric layer between the firstelectrode layer and the second electrode layer. The bending of thepiezoelectric layer 126 flexes the membrane 123 which in turnpressurizes the fluid in the pumping chamber 114 to controllably forcefluid through the descender 116 and eject drops of fluid out of thenozzle 118. Thus, each flow path with its associated actuator providesan individually controllable fluid ejector unit.

In this example, the first electrode layer is the reference electrodelayer 124. The reference electrode layer 124 contains one or morereference electrodes. A reference electrode can be continuous andoptionally can span multiple actuators. A continuous reference electrodecan be a single continuous conductive layer disposed between thepiezoelectric layer 126 and the exposed surface of the membrane layer122 (or the exposed surface of the monolithic module substratecontaining the membrane 123). The membrane 123 isolates the referenceelectrode layer 124 and the piezoelectric layer 126 from the fluid inthe pumping chamber 114. The drive electrode layer 128 is on theopposing side of the piezoelectric layer 126 from the referenceelectrode layer 124. The drive electrode layer 128 includes patternedconductive pieces serving as the drive electrodes for the piezoelectricactuator structure 120.

The drive electrode and the reference electrode are electrically coupledto a controller 130 which supplies a voltage differential across thepiezoelectric layer 126 at appropriate times and for appropriatedurations in a fluid ejection cycle. A voltage differential between afirst electrode and a second electrode is a measure of the voltageapplied to the first electrode (the so-called “drive electrode”)relative to the voltage applied to the second electrode (the so-called“reference electrode”). Where the reference electrode is connected toground, the drive voltage differential between the drive electrode andthe reference electrode is equal to the voltage applied to the driveelectrode, and the terms “drive voltage” and “drive voltagedifferential” are used interchangeably. Typically, electric potentialson reference electrodes are held constant, or are commonly controlledwith the same voltage waveform across all actuators, during operation,e.g., during the firing pulse.

A negative voltage differential exists when the applied voltage on thedrive electrode is lower than the applied voltage on the referenceelectrode. A positive voltage differential exists when the appliedvoltage on the drive electrode is higher than the applied voltage on thereference electrode. In such implementations, the “drive voltage” or“drive voltage pulse” applied to the drive electrode is measuredrelative to the voltage applied to the reference electrode in order toachieve the desired drive voltage waveforms for piezoelectric actuation.

The compliance of the membrane 123 is such that it allows actuation ofthe piezoelectric layer 126 to flex the membrane 123 over the pumpingchamber 114 and pressurize the fluid in the pumping chamber 114.

The piezoelectric layer 126 can include a substantially planarpiezoelectric material, such as a lead zirconium titanate (“PZT”) film.The thickness of the piezoelectric material is within a range thatallows the piezoelectric layer to flex in response to an appliedvoltage. For example, the thickness of the piezoelectric material canrange from about 0.5 to 25 microns, such as about 1 to 7 microns. Thepiezoelectric material can extend beyond the area of the membrane 123over the pumping chamber 114. The piezoelectric material can spanmultiple pumping chambers in the module substrate. Alternatively, thepiezoelectric material can include cuts in regions that do not overliethe pumping chambers, in order to segment the piezoelectric material ofthe different actuators from each other and reduce cross-talk.

The thin film of piezoelectric material can be deposited on the membranelayer 122 by sputtering. Types of sputter deposition can includemagnetron sputter deposition (e.g., RF sputtering), ion-beam sputtering,reactive sputtering, ion-assisted deposition, high-target-utilizationsputtering, and high power impulse magnetron sputtering. Sputteredpiezoelectric material (e.g., piezoelectric thin film) can have a largeas-deposited polarization. Some environments that are used forsputtering piezoelectric material apply a direct current (DC) fieldduring sputtering. The DC field causes the piezoelectric material to bepolarized (or “poled”) in the orientation that results when a negativevoltage is applied to the exposed side of the piezoelectric material. Inthe configuration shown in FIG. 1, the poling direction of thepiezoelectric layer produced using such methods points from thereference electrode layer 124 toward the drive electrode layer 128,e.g., substantially perpendicular to the planar piezoelectric layer 126.

Once the piezoelectric material has been poled, application of anelectric field across the piezoelectric material will displace thepiezoelectric material. For example, a negative voltage differentialbetween the drive electrode and the reference electrode in FIG. 1results in an electric field in the piezoelectric layer 126 that pointssubstantially in the same direction as the poling direction. In responseto the electric field, the piezoelectric material between the driveelectrode and the reference electrode expands vertically and contractslaterally, causing the piezoelectric film over the pumping chamber toflex. Alternatively, a positive voltage differential between the driveelectrode and the reference electrode in FIG. 1 results in an electricfield within the piezoelectric layer 126 that points in a directionsubstantially opposite to the poling direction. In response to theelectric field, the piezoelectric material between the drive electrodeand the reference electrode contracts vertically and expands laterally,causing the piezoelectric film over the pumping chamber to flex in theopposite direction. The direction and shape of the deflection depends onthe shape of the drive electrode and the natural bending mode of thepiezoelectric film that spans beyond the membrane over the pumpingchamber.

Designs and optimizations of the drive electrodes to improve volumedisplacement in the pumping chamber are discussed in further detail inthe sections that follow.

Central Electrode

FIG. 2A is a schematic cross-sectional view of the exemplary fluidejection system 100 with a central drive electrode 210 in the driveelectrode layer. FIG. 2B is a schematic top-view of the same exemplaryfluid ejection system 100 shown in FIG. 2A.

The exemplary fluid ejection system 100 in FIGS. 2A-2B includes themodule substrate 110 with the embedded pumping chamber 114. The pumpingchamber 114 is connected to the flow path between the fluid inlet andthe nozzle (not shown). The top side of the pumping chamber 114 iscovered by the membrane 123. In this example, the membrane has a lateraldimension, i.e., parallel to the flat surface of the substrate 110, D.

The piezoelectric actuator structure is placed over and supported on themembrane 123. The piezoelectric material in the actuator is poled in adirection pointing from the bottom surface to the top surface of thepiezoelectric layer 126. The drive electrode layer includes a centraldrive electrode 210 in contact with the top surface of the piezoelectriclayer 126. The central drive electrode is a single conductive piece(e.g., a disc) that has a lateral dimension, Rc. In this singleelectrode design, the central drive electrode 210 is positioned over thepumping chamber in exclusion of any other drive electrode in the driveelectrode layer. In some implementations, the central drive electrode ispreferably placed over the central portion of the membrane to achievemaximum deflection. The lateral dimension of the central drive electrodeRc can vary from a fraction of D (e.g., ⅔ of D or ¾ of D) to slightlygreater than D (e.g., up to about 10% larger than D). The referenceelectrode layer includes a reference electrode 220 disposed between thepiezoelectric layer 126 and the top surface of the membrane layer 122.The reference electrode can span at least the area projected by thedrive electrode (i.e., the central drive electrode 210). In someimplementations, the reference electrode can also span the driveelectrodes of multiple actuators over multiple pumping chambers.

The piezoelectric actuator is controlled by the controller 130 which iselectrically coupled to the drive electrode 210 and the referenceelectrode 220. The controller 130 can include one or more waveformgenerators that supply appropriate voltage pulses to the drive electrode210 to deflect the actuator membrane in a desired direction during adroplet ejection cycle. The controller 130 can further be coupled to acomputer or processor for controlling the timing, duration, and strengthof the drive voltage pulses.

When using a central drive electrode such as that shown in FIGS. 2A-2B,a negative voltage differential applied across the piezoelectric layergenerates an electric field, E, that points substantially in the samedirection as the poling direction, P. The electric field forms primarilyin the piezoelectric material beneath the drive electrode and in thecentral portion of the membrane over the pumping chamber 114. Inresponse the electric field, the piezoelectric material expandsvertically and contracts laterally. As a result, the central portion ofthe membrane tends to form a concave shape (from the actuator side) andthus bows inwardly (toward the chamber), contracting the pumping chamber114. Alternatively, a positive voltage differential applied across thepiezoelectric material generates an electric field that points in adirection opposite to the poling direction of the piezoelectricmaterial. In response to the electric field, the piezoelectric materialcontracts vertically and expands laterally. As a result, the centralportion of the membrane tends to form a convex shape (from the actuatorside) and thus bows outwardly (away from the chamber), expanding thepumping chamber 114.

FIG. 2C is an exemplary drive voltage waveform for a droplet ejectioncycle in the fluid ejection system of FIGS. 2A-2B. In general, during afluid ejection cycle, the pumping chamber first expands to draw in fluidfrom the fluid supply, and then contracts to eject a fluid droplet fromthe nozzle. Therefore, when using the fluid ejection system utilizing acentral drive electrode (such as the one in FIGS. 2A-2B) and a referenceelectrode, the fluid ejection cycle includes first applying a positivevoltage pulse to the drive electrode to expand the pumping chamber 114and then applying a negative voltage pulse to the drive electrode tocontract the pumping chamber 114. Alternatively, as shown in FIG. 2C, asingle positive voltage pulse of magnitude V and duration T1 is appliedto the drive electrode to expand the pumping chamber and draw in thefluid, and at the end of the pulse, the pumping chamber contracts fromthe expanded state back to a relaxed state and ejects a fluid drop.

Expanding the pumping chamber from a relaxed state using a central driveelectrode requires a positive voltage differential being applied acrossthe piezoelectric layer between the central drive electrode and thereference electrode. One drawback with using such a positive drivevoltage differential is that the electric field generated in thepiezoelectric layer points in a direction opposite to the polingdirection of the piezoelectric material. Repeated application of thepositive voltage differential will cause depolarization of thepiezoelectric layer and reduce the effectiveness of the actuator overtime.

To avoid using a positive drive voltage differential, the driveelectrode can be maintained at a quiescent negative bias relative to thereference electrode, and can be restored to neutral only during theexpansion phase of the fluid ejection cycle. FIG. 2D illustrates anexemplary drive voltage waveform that utilizes a quiescent negativebias. In this alternative, the pumping chamber is kept at apre-compressed state by the quiescent negative bias on the central driveelectrode while idle. During a fluid ejection cycle, the negativevoltage bias is removed from the central drive electrode for a timeperiod T1, and then reapplied until the start of the next fluid ejectioncycle. When the negative bias is removed from the central driveelectrode, the pumping chamber expands from the pre-compressed state tothe relaxed state and draws in fluid from the inlet. After the timeperiod T1, the negative bias is reapplied to the central drive electrodeand the pumping chamber contracts from the relaxed state to thepre-compressed state and ejects a droplet from the nozzle. Thisalternative drive method eliminates the need to apply a positive voltagedifferential between the drive electrode and the reference electrode.However, prolonged exposure to a negative quiescent bias and constantinternal stress can cause deterioration of the piezoelectric material.

An alternative design utilizing a ring-shaped drive electrode avoids theuse of either a positive drive voltage differential or a quiescentnegative bias on the drive electrode relative to the referenceelectrode. Details of the ring-shaped drive electrode design aredescribed in the sections that follow.

Ring Electrode

FIG. 3A is a schematic cross-sectional view of an exemplary fluidejection system 100 with a ring-shaped drive electrode 310. FIG. 3B is aschematic top view of the same exemplary fluid ejection system 100 shownin FIG. 3A.

The exemplary fluid ejection system 100 in FIGS. 3A-3B includes themodule substrate 110 with the embedded pumping chamber 114. The pumpingchamber 114 is connected to the flow path between the fluid inlet andthe nozzle (not shown). In some implementations, the top side of thepumping chamber 114 is covered by the membrane 123. The membrane has alateral dimension, D.

The piezoelectric actuator structure is placed over and supported on themembrane. The piezoelectric material in the actuator is poled in adirection pointing from the bottom surface to the top surface of thepiezoelectric layer 126, i.e., substantially perpendicular to themembrane. The drive electrode layer includes a ring-shaped driveelectrode 310 in contact with the top surface of the piezoelectric layer126.

The ring-shaped drive electrode 310 is a conductive piece that has anaperture or opening in the middle (e.g., a circular ring). Thering-shaped drive electrode can be characterized by an outer shape ofthe conductive piece and an inner shape of the opening (or aperture) inthe conductive piece. The inner shape and the outer shape do not have tobe geometrically similar or concentric. However, in some implementationsthe width of the ring can be roughly uniform around the entirety of thering or most part of the ring.

In this single electrode design, the ring-shaped drive electrode 310 ispositioned over the pumping chamber in exclusion of any other driveelectrode in the drive electrode layer. The opening of the ring-shapeddrive electrode is placed over the central portion of the membrane overthe pumping chamber 114. The conducting portion of the ring-shaped driveelectrode is placed over the peripheral portion of the membrane. Theperimeter 315 of the membrane over the pumping chamber is entirelycovered by the ring-shaped electrode 310 as shown in FIGS. 3A-3B. InFIGS. 3A-3B, the outer and inner edge of the ring-shaped drive electrodeis aligned with (i.e., parallel to) the perimeter of the membrane overthe pumping chamber.

The ring-shaped drive electrode 310 can be characterized by an innerlateral dimension R_(in) for the inner shape of the opening and an outerlateral dimension R_(out) for the outer shape of the conductive piece.The inner lateral dimension of the ring-shaped drive electrode R_(in),can range from a fraction of D, e.g., ⅔ of D or ¾ of D, to slightly lessthan the outer lateral dimension R_(out). The outer lateral dimension ofthe ring-shaped drive electrode R_(out), is at least D, and can begreater than D, e.g., about 10% greater than D. In some implementations,for a pumping chamber with a lateral dimension of roughly 150 microns,the outer lateral dimension of the ring-shaped electrode is about 5-10microns greater than the lateral dimension of the pumping chamber.

The reference electrode layer includes a reference electrode 220disposed between the piezoelectric layer 126 and the top surface of themembrane layer 122. The reference electrode 220 can span at least thearea projected by the top drive electrode (i.e., the ring-shaped driveelectrode 310). In some implementations, the reference electrode canspan beyond the area covered by the top drive electrode to cover thecentral region enclosed by the ring-shaped drive electrode. In someimplementations, the reference electrode can also span one or moreactuators.

The piezoelectric actuator is controlled by the controller 130 which iselectrically coupled to the drive electrode 310 and the referenceelectrode 220. The controller 130 can include one or more waveformgenerators that supply appropriate voltage pulses to the drive electrode210 to deflect the actuator membrane in a desired direction during adroplet ejection cycle. The controller 130 can further be coupled to acomputer or processor for controlling the timing, duration, and strengthof the voltage pulses.

When using the ring-shaped drive electrode and the reference electrodesuch as those shown in FIGS. 3A-3B, a negative voltage differentialapplied across the piezoelectric layer generates an electric field thatpoints substantially in the same direction as the poling direction. Theelectric field forms primarily in the piezoelectric material underneaththe ring-shaped drive electrode and over the peripheral portion of themembrane. In response to the electric field, the piezoelectric materialunder the ring-shaped drive electrode expands vertically and contractslaterally. As a result, the central portion of the membrane tends toform a convex shape (from the actuator side) and thus bows outwardly(away from the chamber), expanding the pumping chamber 114.Alternatively, a positive voltage differential applied across thepiezoelectric layer generates an electric field that points in adirection opposite to the poling direction. In response to the electricfield, the piezoelectric material under the drive electrode contractsvertically and expands laterally. As a result, the central portion ofthe membrane tends to form a concave shape (from the actuator side) andthus bows inwardly (toward the chamber), contracting the pumping chamber114.

The deflection of the membrane under a ring-shaped drive electrode is inthe opposite direction as that under a central drive electrode if thesame voltage differential is applied across the piezoelectric layerbetween the drive electrode and the reference electrode. This is due tothe natural bending mode that exists in a piezoelectric film. Althoughthe local behavior of the piezoelectric film underneath a ring-shapeddrive electrode is similar to that underneath a central drive electrode,by using a piezoelectric film that extends beyond the area covered bythe drive electrode, additional deflection is created in thepiezoelectric material over the central region of the membrane due tothe tensions created underneath the drive electrode. Such deflectionwould not naturally occur if the piezoelectric layer is segmented at theinner edge of the ring-shaped electrode.

FIG. 3C is an exemplary drive voltage waveform for a droplet ejectioncycle in the fluid ejection system of FIGS. 3A-3B. Because a ring-shapeddrive electrode creates the opposite deflection as a central driveelectrode, a negative drive voltage differential can be used to replacethe positive voltage differential in FIG. 2C and achieve the same fluidejection cycle in the pumping chamber. In addition, there is also noneed to maintain a quiescent negative bias on the drive electrode toachieve a pumping action. During a fluid ejection cycle in the fluidejection system 100 as shown in FIGS. 3A-3B, a negative voltage pulse ofmagnitude −V is first applied to the ring-shaped drive electrode 310 fora time period T1, the pumping chamber 114 expands to draw in fluid fromthe fluid supply, then at the end of the negative voltage pulse, thepumping chamber 114 contracts and goes from the expanded state back to arelaxed state, ejecting a fluid drop from the nozzle.

A ring-shaped drive electrode solves the aforementioned drawbacks of thecentral drive electrode. A negative voltage pulse can be used as drivesignals for fluid ejection, and there is no exposure to a quiescentbias, therefore, the ring-shaped drive electrode can improve the lifespan of the piezoelectric material in the actuator.

FIG. 4A illustrates the deflection of a piezoelectric membrane under apositive voltage differential applied between a circular central driveelectrode and a reference electrode.

FIG. 4B illustrates the deflection of a piezoelectric membrane under apositive voltage differential applied between an annular ring-shapeddrive electrode and a reference electrode.

In FIG. 4A, a positive voltage differential of 1V is applied across thecentral portion of the circular piezoelectric membrane while the edge isheld at ground. The radius of the pumping chamber is 150 microns and thetotal central displacement is approximately 7.1 nm, and the total volumeexpansion is 142×10¹⁸ m³.

In FIG. 4B, a positive voltage differential of 1V is applied across theedge of the piezoelectric membrane while the central region is held atground. The radius of the pumping chamber is also 150 microns, and thetotal central displacement is approximately −4.6 nm, and the totalvolume contraction is 98×10¹⁸ m³.

Dual Electrode Design with a Central Electrode and a Ring Electrode

A single central drive electrode and a single ring-shaped driveelectrode can be combined to create a dual electrode design. A unipolar(e.g., negative) voltage pulse at zero quiescent bias can be applied tothe ring-shaped drive electrode and the central drive electrodeconsecutively, to first expand and then contract the pumping chamber.Unlike the single electrode design shown in FIGS. 2A-3C, the pumpingvoltage differential is effectively reduced in a dual electrode designbecause the total volume displacement required for a pumping cycle isdivided between the expansion phase and the contraction phase. Inaddition, for a given drive voltage differential, the size of thepumping chamber under the dual-electrode can be significantly smallerthan that in a single electrode design because of the larger membranedeflection created by the dual-electrode. Therefore, a higher density ofpumping chambers and nozzles can be packed in a printhead module thatutilizes the dual electrode design, creating higher printing resolution.

FIG. 5A is a schematic diagram of a dual-electrode piezoelectricactuator including an inner drive electrode (e.g., a central electrode),an outer drive electrode (e.g., a ring-shaped electrode) encircling theinner drive electrode, and an opposing reference electrode.

In FIG. 5A, the inner drive electrode 510 and the outer drive electrode520 are insulated from each other by a gap between the electrodes. Inthis example, the inner drive electrode 510 is a circular conductivedisc, and the outer drive electrode 520 is a conductive annular ring.The lateral dimension R_(c) of the inner drive electrode is smaller thanthe inner lateral dimension of the outer drive electrode R_(in). Theouter dimension of the outer drive electrode R_(out) is greater or equalto the lateral dimension of the membrane over the pumping chamber.

The inner and outer drive electrodes are positioned on the top surfaceof the piezoelectric layer 126. The inner drive electrode 510 isdisposed over a central portion of the membrane over the pumpingchamber. The outer drive electrode 520 is disposed over a peripheralportion of the membrane over the pumping chamber. The outer driveelectrode covers the perimeter of the membrane. The inner and outeredges of the outer drive electrode 520 are aligned and spaced apart fromthe pumping chamber wall.

The bottom surface of the piezoelectric layer 126 is in contact with areference electrode 530. The bottom surface of the reference electrode530 is in contact with the actuator membrane 122. The referenceelectrode 530 can be continuous and spans both the inner and the outerdrive electrodes. The reference electrode 530 can also span driveelectrodes of multiple actuators. Alternatively, the reference electrodecan also be segmented into inner and outer portions that correspond tothe inner and outer drive electrodes, and the portions can be separatelycontrollable. The inner portions of the reference electrodes of multipleactuators can be electrically connected to a first common referencevoltage and commonly controlled, and the outer portions of the referenceelectrodes of multiple actuators can be electrically connected to asecond common reference voltage and commonly controlled (but separatelyfrom the inner portions).

The inner drive electrode 510, the outer drive electrode 520, and thereference electrode 530 are coupled to a controller 130. The controller130 can supply the appropriate drive voltages for actuating thepiezoelectric membrane over the pumping chamber. In someimplementations, the outer drive electrode 520 can have one or moresmall slits that cut across the width of the ring to allow access to theinner drive electrode 510. For example, a slit across the width of theouter drive electrode 520 can be used to accommodate a conductive pathbetween the inner drive electrode 510 and the controller 130. Theexistence of a slit will not substantially affect the operation of thedrive electrodes if the width of the slit is small compared to the sizeof the outer drive electrode, and the outer drive electrode can stillsubstantially enclose the inner drive electrode 510. Alternatively, aninsulated conductive path can be placed over or under the outer driveelectrode 520 to connect to the inner electrode 510 to the controller130.

In this particular example, the dual electrode piezoelectric actuatorincludes a piezoelectric layer made of a PZT film (e.g., a continuousPZT film that spans beyond the membrane). The PZT film is approximately5 microns thick, and has a poling direction pointing from the bottomsurface to the top surface, i.e., substantially perpendicular to themembrane. The thickness for the PZT film can range from about 0.5 micronto 10 microns. The membrane layer is a silicon membrane approximately 11micron thick. The flexible portion of the membrane layer forms themembrane over the pumping chamber. The pumping chamber and the membraneeach has a lateral dimension of 150 microns. The inner drive electrodehas a radius of roughly two thirds (⅔) of the lateral dimension of thepumping chamber. The outer electrode has an outer dimension that is atleast equal to or greater than the lateral dimension of the pumpingchamber (or the lateral dimension of the membrane).

FIG. 5B illustrates exemplary drive voltage waveforms on the inner driveelectrode and the outer drive electrode relative to the referenceelectrode (i.e., a continuous reference electrode that spans both theinner and the outer electrodes in this example) for a droplet ejectioncycle in the fluid ejection system utilizing the dual electrodepiezoelectric actuator shown in FIG. 5A.

In the dual electrode design, the drive electrodes are kept neutralduring idle. No quiescent bias is applied to either drive electroderelative to the reference electrode. During a fluid ejection cycle, afirst negative voltage pulse is applied to the outer drive electrode toexpand the pumping chamber during a time period T1; then a secondnegative voltage pulse is applied to the inner drive electrode tocontract the pumping chamber during a time period T2. FIG. 5Billustrates the correspondence between the drive voltages applied to thedrive electrodes relative to the reference electrode and the shape ofthe membrane during the pumping cycle. For the same drive voltagemagnitude V, the total volume displacement caused by the dual electrodeactuator is the sum of the total volume displacements of the inner driveelectrode and the outer drive electrode under the same drive voltages.In some implementations, the drive voltage applied on the inner driveelectrode and the outer drive electrode can be of the same or differentmagnitudes depending on the particular design and/or operationrequirements.

The drive voltage pulses can be supplied by a controller 130electrically coupled to the inner and outer drive electrodes, and thereference electrode. The controller 130 is operable to generate orobtain a waveform for the negative voltage pulses. The controller canreceive or generate control signals that control the timing, duration,magnitude, and time lag of the drive pulses on each electrode. Theduration of the drive voltage pulse can be determined based on the timeneeded for the actuator membrane to reach maximum deflection. The timelag between the first drive voltage pulse and the second drive voltagepulse can be determined based on the time it takes the membrane toreturn to its relaxed state from the expanded state.

Optimization and Variations of the Piezoelectric Actuator

The relative size of the drive electrode and the pumping chamber affectsthe total volume displacement that can be achieved in both the singleelectrode designs (a central drive electrode or a ring-shaped driveelectrode) and the dual electrode design (an inner drive electrode andan outer drive electrode surrounding the inner drive electrode). Themaximum total volume displacement can be achieved for an electrode of aparticular shape by adjusting the electrode size.

FIG. 6 is a plot illustrating the correlations between volumedisplacement per volt and the lateral dimensions of the central and thering-shaped drive electrodes in relation to the lateral dimension of thepumping chamber. In this plot, the dimension of the pumping chamber isidentical to the dimension of the membrane over the pumping chamber(i.e., the walls of the pumping chamber are perpendicular to themembrane).

These plots are for electrodes of circular shapes (i.e., a circular discor an annual ring) placed at the center of the actuator membrane over apumping chamber. The pumping chamber has a radius of 150 microns. Theactuator membrane is 11 microns thick. The piezoelectric layer is 5microns thick. Although the exact values shown in the plot are specificto this particular configuration, the correlations between the volumedisplacement and electrode size are illustrative of electrodes andpumping chambers of other sizes and shapes.

The horizontal axis of the plot shows the distance from the center ofthe pumping chamber. The maximum distance shown is 150 microns, theradius of the pumping chamber. The vertical axis on the left is thecapacitance (F) of the electrodes. The vertical axis on the right is thevolume displacement per volt (m³/volt). Curve 601 is the correlationbetween the capacitance of a circular central electrode and the radiusof the circular central electrode. Curve 602 is the correlation betweenthe capacitance of a circular ring-shaped electrode and the inner radiusof the circular ring-shaped electrode. Curve 603 is the sum of the Curve601 and the Curve 602. Curve 604 is the correlation between the volumedisplacement of a circular central electrode and the radius of thecircular central electrode. Curve 605 is the correlation between thevolume displacement of a circular ring-shaped central electrode andinner radius of the circular ring-shaped electrode. Curve 606 is the sumof the Curves 604 and 605.

Curve 604 shows that a positive drive voltage on a central electrodeproduces a positive volume displacement (i.e., chamber expands). Themaximum expansion occurs as the radius of the circular electrode reachesabout 100 microns, which is about two thirds (⅔) of the pumping chamberradius. Before the radius of the central electrode reaches two thirds(⅔) of the pumping chamber radius, the volume displacement increaseswith electrode radius; after the radius reaches ⅔ of the pumping chamberradius, the volume displacement starts to decrease with electroderadius. Although FIG. 6 only shows electrode radius up to the pumpingchamber radius, it can be seen from the non-zero volume displacement atpumping chamber radius that electrode radius that goes slight beyond thepumping chamber radius can still produce volume displacement in thepumping chamber.

Curve 605 shows that a positive drive voltage on a ring-shaped driveelectrode produces a negative volume displacement (i.e., the chambercontracts). The maximum contraction occurs as the inner radius of thering-shaped electrode reaches about 112 microns. The out radius of thering-shaped drive electrode is the same as the pumping chamber radius inthis plot. However, the outer radius can be, and preferably is, slightlygreater than the pumping chamber radius. For example, for a pumpingchamber with a radius of 150 microns, the outer radius of thering-shaped drive electrode can be 155-160 microns.

It can be seen that the ring-shaped electrode is slightly less efficientthan the central electrode regarding maximum volume displacement. Forexample, the maximum volume displacement for the circular centralelectrode is roughly 1.9×10⁻¹⁶ m³/volt at radius, r_(c1)=100 microns.The maximum volume displacement for the circular ring-shaped electrodeis roughly 1.7×10⁻¹⁶ m³/volt at inner radius, r_(re)=112 microns. In onedual electrode design, the radius of the central drive electrode and theinner radius of the ring-shaped drive electrode are both chosen toachieve the maximum volume displacement in the pumping chamber. In thisparticular example, such design requires approximately a 12-micron gapbetween the central and the ring-shaped drive electrodes.

In an alternative dual electrode design, the radius for the inner driveelectrode (e.g., the central drive electrode) and the inner radius ofthe outer drive electrode (e.g., the ring-shaped drive electrode) aresuch that the volume displacement is equal for the two drive electrodes.If the inner radius of the ring-shaped drive electrode is r_(re), theradius of the central drive electrode is r_(c2)=80 microns. The volumedisplacement per volt would be the same in magnitude for the ring-shapeddrive electrode and the central drive electrode at 1.7×10⁻¹⁶ m³/volt. Inthis design, there is approximately a 32 micron gap between theelectrodes.

In addition to the relative sizes of the pumping chamber and the driveelectrodes, the maximum volume displacement is also affected slightly bythe thickness of the piezoelectric layer. FIG. 7 is a plot illustratingthe correlation between volume displacement per volt (dV/dU) in thepumping chamber and percentage of a central drive electrode radiusrelative to pumping chamber radius (%). In general, the volumedisplacement per volt peaks when the radius of the central driveelectrode reaches about ⅔ of the pumping chamber radius. However,depending on the thickness of the PZT film, the peak shifts slightly.For example, with a thicker PZT film, the peak shifts to a slightlysmaller electrode radius, and for a thinner PZT film, the peak shifts toa slightly larger electrode radius. In general, the volume displacementper volt reduces as the PZT film thickness increases. A possible designregime for the central drive electrode is about 60% to 80% of thepumping chamber radius. The thickness of the PZT film can range from 0.5micron to 10 microns to maintain both sufficient flexibility for volumedisplacement and sufficient strength to pressurize the fluid in thepumping chamber.

Although the plots in FIGS. 6 and 7 are for a circular inner electrodeand/or an annular ring-shaped outer electrode, similar dimensionalcorrelations can be found for electrodes of other shapes. For example,the ring shape for the outer electrode is not limited to circular butrather can include other shapes (e.g., oval, polygonal, square,rectangular, triangular, and so on) with an open center. In addition, insome implementations, a ring shape can include a gap cut across thewidth of the ring (e.g., to accommodate a path connecting the innerelectrode to the controller). In some implementations, a ring-shapeddrive electrode can have more than one slit cut across the width of thering such that it is no longer a single conductive piece. In suchimplementations, the several segments of the ring-shaped electrode canbe tied to a common control signal or controlled synchronously to act asa single ring-shaped drive electrode. In such implementations, suchslits do not substantially alter the effectiveness of the ring-shapeddrive electrode if the width and/or number of the slits are smallrelative to the size of the ring, such that the segments of thering-shaped electrode still substantially enclose the central regionsurrounded by the segments of the ring-shaped drive electrode.

Similarly, the inner drive electrode is not limited to a circular disc,and can have any shape (e.g., oval, polygonal, square, rectangular, ortriangular, and so on). In some implementations, the inner driveelectrode can include one or more cuts so long as the cuts are smallcompared to the area of the inner drive electrode and all segments ofthe inner electrode are tied to a common control signal or controlledsynchronously to act as a single central drive electrode. In someimplementations, the inner drive electrode can include one or moreopenings in it (e.g., a ring, an open ring, or a disc with holes in it).The presence of the one or more openings do not substantially interferewith the operation of the inner drive electrode if the openings aresmall in number and size relative to the size of the inner driveelectrode.

Positions of the drive electrodes can also affect the performance of thepiezoelectric actuator. The central and/or inner drive electrode can bedisposed generally over the central portion of the membrane over thepumping chamber. In the designs shown in FIGS. 2-5, the circular centralor inner drive electrodes are disposed at the center of the membraneover pumping chamber. The location of the central or inner driveelectrode can be varied slightly in some implementations (e.g., toaccommodate shape of the pumping chamber or positions of othercomponents of the fluid ejector unit). Typically, by placing the innerdrive electrode over the central portion of the membrane, greaterdeflection of the membrane can be achieved. However, some off-set fromthe central portion may be desirable depending on the particular designof the drive electrode and the shape of the pumping chamber.

Similarly, the ring-shaped and/or the outer drive electrode can bedisposed over the peripheral portion of the membrane with its openinggenerally over the central portion of the membrane. In the designs shownin FIGS. 3-5, the ring-shaped and/or outer drive electrode have innerand outer edges substantially aligned with the perimeter of the pumpingchamber wall and covers the entire (or substantially the entire)periphery of the pumping chamber. Such aligned positioning of thering-shaped and/or outer drive electrode can allow greater membranedeflection than the unaligned positioning.

The inner and outer perimeters of the ring-shaped electrode do not haveto be geometrically similar or concentric. However, in someimplementations, the width of the ring is roughly uniform around theentirety of the ring or most of the ring. Similarly, the outer perimeterof the inner electrode and the inner perimeter of the ring-shapedelectrode do not have to be geometrically similar or concentric.However, in some implementations, the gap between the inner electrodeand the outer electrode can be roughly uniform around the entirety ofthe inner electrode.

The reference electrode can be segmented between the portions eachopposing a respective inner or the outer drive electrode of an actuator.The reference electrode can also be continuous and span both the innerand the outer drive electrodes of an actuator. In some designs, acontinuous reference electrode can span electrodes of multipleactuators. In some designs, reference electrodes of multiple actuatorsor segmented portions of a reference electrode for a single actuator canbe tied to a common control signal or controlled synchronously.

The piezoelectric material can be continuous and span over the entirearea over the pumping chamber and beyond. In some implementations, thecontinuous piezoelectric material can span several actuators.Alternatively, the piezoelectric material can include cuts in regionsthat do not overlie the pumping chambers, in order to segment thepiezoelectric material of the different actuators from each other andreduce cross-talk.

FIG. 8A shows a piezoelectric actuator with another exemplaryring-shaped electrode 800 over a continuous PZT layer. FIG. 8Billustrates the deflection of the PZT layer under the exemplaryring-shaped electrode 800 in FIG. 8A.

In this design, the shape of the ring is defined by an inner shapeformed by the inner edge 810 of the ring, and an outer shape formed bythe outer edge 820 of the ring. The inner shape of the ring is roughly ahexagon, except one apex of the hexagon that is connected to theexternal voltage source or controller 130 is slightly elongated androunded. The outer shape of the ring is similar to that of the innershape, and the inner shape and the outer shape are roughly concentricsuch that the width (W) of the ring is roughly the same around the ring,except for the side that connects to the voltage source or controller130.

The ring-shaped electrode 800 in FIG. 8A is placed on the top surface ofa continuous piezoelectric film 840 and over a pumping chamber. Thepumping chamber edge 830 is entirely underneath the ring-shapedelectrode 800. The shape of the ring also tracks the shape of thepumping chamber closely. Therefore, the ring is substantially alignedwith the perimeter of the membrane over the pumping chamber. Acontinuous reference electrode is disposed between the piezoelectricfilm 840 and the silicon substrate 850.

FIG. 8B shows a simulation result for the deflection of thepiezoelectric (e.g., PZT) film when a negative voltage is applied to thering-shaped drive electrode 800 relative to the reference electrode. Themembrane bow outwardly (away from the pumping chamber), and the pumpingchamber is expanded.

The design of the inner and outer drive electrodes in a dual electrodedesign and the design for the ring-shaped electrode in a singleelectrode design can also take into consideration that the static anddynamic deflection for the piezoelectric membrane at 0 Hz and at thepumping chamber mechanical resonance frequency should be roughly thesame.

Although the drive voltages discussed above are simple rectangularvoltage pulses, much more complex voltages can be applied, e.g., inorder to control the size, velocity or number of fluid drops ejected.

Although the piezoelectric material discussed with respect to the aboveexamples has a poling direction pointing away from the pumping chamber.In some implementations, a prefixed piezoelectric layer (e.g., bulkpiezoelectric material that is fired before being bonded to a modulesubstrate) having a poling direction pointing toward the pumping chambercan be attached to the membrane. In such implementations, the voltagepulses could be similarly reversed to achieve the same pumping actions.

Although the discussions above refer to a configuration where the driveelectrode layer is placed over the exposed side of the piezoelectriclayer and the reference electrode layer is disposed between thepiezoelectric layer and the membrane layer, the positions of the driveelectrode and the reference electrode layers can be reversed as well.For a given a poling direction of the piezoelectric material relative tothe pumping chamber (i.e., either pointing away or toward the pumpingchamber), the voltage pulses applied to the drive electrodes relative tothe reference electrode could be similarly reversed to achieve the samepumping actions.

The selection of appropriate pumping voltage pulses on each driveelectrode in each configuration can be based on the following basicbehaviors of the outer (e.g., ring-shaped) drive electrode and the inner(e.g., central) drive electrode.

The basic behaviors of a ring-shaped drive electrode are as follows:when a voltage differential applied between the ring-shaped driveelectrode and the reference electrode creates an electric field thatpoints in the same direction as the poling direction of thepiezoelectric material between the ring-shaped drive electrode and thereference electrode, the pumping chamber expands; when a voltagedifferential applied between the ring-shaped drive electrode and thereference electrode creates an electric field that points in a directionopposite to the poling direction of the piezoelectric material betweenthe ring-shaped drive electrode and the reference electrode, the pumpingchamber contracts.

The basic behaviors of a central drive electrode are as follows: when avoltage differential applied between the central drive electrode and thereference electrode creates an electric field that points in the samedirection as the poling direction of the piezoelectric material betweenthe central drive electrode and the reference electrode, the pumpingchamber contracts; when a voltage differential applied between thecentral drive electrode and the reference electrode creates an electricfield that points in a direction opposite to the poling direction of thepiezoelectric material between the central drive electrode and thereference electrode, the pumping chamber expands.

There are four situations where the voltage differential between a driveelectrode and a reference electrode can create an electric field thatpoints in the same direction as the poling direction of thepiezoelectric material.

First, when the poling direction of the piezoelectric material pointsaway from the pumping chamber, if the electrodes are positioned suchthat the poling direction also points from the reference electrode layerto the drive electrode layer, then a negative voltage differentialbetween the drive electrode and the reference electrode creates anelectric field that points in the same direction as the poling directionof the piezoelectric material.

Second, when the poling direction of the piezoelectric material pointsaway from the pumping chamber, if the electrodes are positioned suchthat the poling direction points from the drive electrode layer to thereference electrode layer, then a positive voltage differential isneeded between the drive electrode and the reference electrode to createan electric field that points in the same direction as the polingdirection of the piezoelectric field.

Third, when the poling direction of the piezoelectric material pointstoward the pumping chamber, if the electrodes are positioned such thatthe poling direction also points from the reference electrode layer tothe drive electrode layer, then a negative voltage differential betweenthe drive electrode and the reference electrode creates an electricfield that points in the same direction as the poling direction of thepiezoelectric material.

Fourth, when the poling direction of the piezoelectric material pointstoward the pumping chamber, if the electrodes are positioned such thatthe poling direction points from the drive electrode layer to thereference electrode layer, then a positive voltage differential isneeded between the drive electrode layer and the reference electrodelayer to create an electric field that points in the same direction asthe poling direction of the piezoelectric field.

There are similarly four situations where the voltage differentialbetween a drive electrode and a reference electrode can create anelectric field that points in a direction opposite to the polingdirection of the piezoelectric material.

During a pumping cycle, it is sometimes preferred to have electricfields that point in the same direction as the poling direction of thepiezoelectric material during both the expansion and the contractionphase (such as in the dual electrode design example). In someimplementations, it is possible to have an electric field pointing inthe same direction as the poling direction only during the expansionphase of the pumping cycle, and the contraction is carried out by thenatural relaxation of the membrane in the absence of an applied electricfield. Application of an electric field that points in the samedirection as the poling direction helps reduce depolarization andfatigue of the piezoelectric material, however, in practice, thispreference may be forgone to achieve other goals.

During a pumping cycle, it is preferable to first expand and thencontract the pumping chamber. When using a single ring-shaped driveelectrode, an electric field pointing in the same direction as thepoling direction can be generated first for a time period T to expandthe pumping chamber, and then eliminated to restore the pumping chamberto its relaxed state. For further contraction, after the expansionperiod, an electric field pointing in a direction opposite to the polingdirection can be applied for a time period T′, although this will tendto deteriorate the piezoelectric material.

When using a dual electrode design with an inner drive electrode and anouter drive electrode, in order to expand the pumping chamber, a firstvoltage differential is applied between the outer drive electrode andthe reference electrode to create an electric field pointing in the samedirection as the poling direction of the piezoelectric material for afirst time period T1; then, to contract the pumping chamber, a secondvoltage differential is applied between the inner drive electrode andthe reference electrode to create an electric field that also points inthe same direction as the poling direction of the piezoelectric materialfor a second time period T2. This sequence of operation reducesdepolarization of the piezoelectric material since the electric fieldcreated in the piezoelectric material points in the same direction asthe poling direction during the entire pumping cycle (e.g., expansionand contraction periods). In some implementations, it is possible toreverse the voltage application, for example, by applying a firstvoltage differential on the inner electrode to generate an electricfield that points in a direction opposite to the poling direction toexpand the chamber first, and then applying a second voltagedifferential on the outer drive electrode to generate another electricfield that points in a direction opposite to the poling direction tocontract the chamber. However, this sequence of operation would increasedepolarization of the piezoelectric material, and reduce the life spanand effectiveness of the piezoelectric actuator.

The use of terminology such as “front” and “back,” “top” and “bottom,”or “horizontal” and “vertical” throughout the specification and claimsis to distinguish the relative positions or orientations of variouscomponents of the printhead module and other elements described therein,and does not imply a particular orientation of the printhead module withrespect to gravity.

While this specification contains many specific implementation details,unless explicitly stated in the claims, these should not be construed aslimitations on the scope of any invention or of what may be claimed, butrather as descriptions of features that may be specific to particularembodiments of particular inventions.

What is claimed:
 1. A fluid ejection system, comprising: a substratehaving a chamber formed therein; a membrane that forms a wall of thechamber and is operable to expand or contract the chamber by flexing;and an actuator supported on the membrane, the actuator comprising: apiezoelectric layer disposed between a drive electrode layer and areference electrode layer, wherein: the piezoelectric layer comprises acontinuous planar piezoelectric material spanning the chamber and havinga uniform poling direction substantially perpendicular to the continuousplanar piezoelectric material; the drive electrode layer comprises aplurality of drive electrodes, the plurality of drive electrodesincluding an inner electrode and an outer electrode surrounding theinner electrode; and the reference electrode layer comprises a referenceelectrode, the reference electrode having a first portion spanning theinner electrode and a second portion spanning the outer electrode, wherecreation of a voltage differential between at least one of the pluralityof drive electrodes and the reference electrode generates an electricfield in the continuous planar piezoelectric material, and the electricfield results in actuation of the continuous planar piezoelectricmaterial to flex the membrane.
 2. The fluid ejection system of claim 1,further comprising: a controller electrically coupled to the pluralityof drive electrodes and the reference electrode, wherein during anoperation cycle to eject a fluid droplet, the controller is operable tocreate a first voltage differential pulse between a first electrode inthe plurality of drive electrodes and the reference electrode during afirst time period to expand the chamber, and to create a second voltagedifferential pulse between a second electrode in the plurality of driveelectrodes and the reference electrode during a second time period tocontract the chamber, the second electrode being different from thefirst electrode and the second time period being after the first timeperiod.
 3. The fluid ejector system for claim 2, wherein: the firstvoltage differential pulse and the second voltage differential pulseeach generates an electric field that points in substantially the samedirection as the poling direction, the first electrode in the pluralityof drive electrode is the outer electrode, and the second electrode inthe plurality of drive electrode is the inner electrode.
 4. The fluidejection system of claim 1, wherein the membrane is a separate layerbonded to the substrate.
 5. The fluid ejection system of claim 1,wherein the inner electrode is disposed over a central portion of themembrane and the outer electrode is disposed over a peripheral portionof the membrane surrounding the inner electrode.
 6. The fluid ejectionsystem of claim 1, wherein the inner electrode and the outer electrodeare the only drive electrodes disposed over the membrane.
 7. The fluidejection system of claim 1, wherein the membrane has a lateral dimensionof D, and the inner electrode has lateral dimension of approximately ⅔of D.
 8. The fluid ejection system of claim 7, wherein the outerelectrode is in a shape of a ring, the ring has an inner lateraldimension and an outer lateral dimension, and the inner lateraldimension of the ring is greater than the lateral dimension of the innerelectrode.
 9. The fluid ejection system of claim 8, wherein the outerlateral dimension of the ring is greater than the lateral dimension ofthe membrane.
 10. The fluid ejection system of claim 9, wherein thelateral dimension of the inner electrode and the inner lateral dimensionof the outer electrode are such that a maximum volume displacement isachieved between expansion and contraction of the chamber.
 11. The fluidejection system of claim 8, wherein the lateral dimension of the innerelectrode and the inner lateral dimension of the outer electrode aresuch that equal volume displacement is achieved between expansion andcontraction of the chamber.
 12. The fluid ejection system of claim 1,wherein the continuous planar piezoelectric material is a lead zirconatetitanate (PZT) film.
 13. A fluid ejection system, comprising: asubstrate having a chamber formed therein; a membrane that forms a wallof the chamber and is operable to expand or contract the chamber byflexing; an actuator supported on the membrane, the actuator comprising:a piezoelectric layer disposed between a drive electrode layer and areference electrode layer, wherein: the piezoelectric layer comprises aplanar piezoelectric material spanning beyond the chamber and having auniform poling direction substantially perpendicular to the planarpiezoelectric material; the drive electrode layer comprises a pluralityof drive electrodes, the plurality of drive electrodes including aninner electrode and an outer electrode surrounding the inner electrode;and the reference electrode layer comprises a reference electrode, thereference electrode having a first portion spanning the inner electrodeand a second portion spanning the outer electrode; and a controllerelectrically coupled to the pair of drive electrodes and the referenceelectrode, wherein during an operation cycle to eject a fluid droplet,the controller is operable to create a first voltage differential pulsebetween a first electrode in the plurality of drive electrodes and thereference electrode during a first time period to expand the chamber,and to create a second voltage differential pulse between a secondelectrode in the plurality of drive electrodes and the referenceelectrode during a second time period to contract the chamber, thesecond electrode being different from the first electrode and the secondtime period being after the first time period.
 14. The fluid ejectorsystem for claim 13, wherein: the first voltage differential pulse andthe second voltage differential pulse each generates an electric fieldthat points substantially in the same direction as the poling direction,the first electrode in the plurality of drive electrode is the outerelectrode, and the second electrode in the plurality of drive electrodeis the inner electrode.
 15. The fluid ejector system of claim 14,wherein: the uniform poling direction of the planar piezoelectricmaterial points from the reference electrode layer to the driveelectrode layer, the first voltage differential pulse is a negativevoltage differential pulse, and the second voltage differential pulse isanother negative voltage differential pulse.
 16. The fluid ejectorsystem of claim 14, wherein: the uniform poling direction of the planarpiezoelectric material points from the drive electrode layer to thereference electrode layer, the first voltage differential pulse is apositive voltage differential pulse, and the second voltage differentialpulse is another positive voltage differential pulse.
 17. The fluidejector system for claim 13, wherein: the first voltage differentialpulse and the second voltage differential pulse each generates anelectric field that points in a direction substantially opposite to thepoling direction, the first electrode in the plurality of driveelectrode is the inner electrode, and the second electrode in theplurality of drive electrode is the outer electrode.
 18. The fluidejector system of claim 17, wherein: the uniform poling direction of theplanar piezoelectric material points from the reference electrode layerto the drive electrode layer, the first voltage differential pulse is apositive voltage differential pulse, and the second voltage differentialpulse is another positive voltage differential pulse.
 19. The fluidejector system of claim 17, wherein: the uniform poling direction of theplanar piezoelectric material points from the drive electrode layer tothe reference electrode layer, the first voltage differential pulse is anegative voltage differential pulse, and the second voltage differentialpulse is another negative voltage differential pulse.